Age-related macular degeneration (AMD) is the primary cause of visual loss in adults over 50 in the developed world. The leading cause of severe vision loss in AMD is choroidal neovascularization (CNV), the growth of pathologic blood vessels under the retina resulting in neovascular AMD (nvAMD). Intra-vitreal injections of anti-VEGF agents have become the mainstay of therapy for nvAMD. However, the long-term response to anti-VEGF therapy is highly variable, with 10 - 20 % of subjects requiring only a few injections to control the CNV activity while over 50% of patients requiring frequent, almost monthly, injections (a condition terms anti-VEGF refractoriness). Understanding the factors underlying the variability in response to anti-VEGF therapy in nvAMD is a critical aspect in designing better treatment for nvAMD. A factor playing a large role in the variability to anti-VEGF therapy may be the proportion of non- endothelial cells (EC) in the CNV as lesions that contain a high proportion of non-ECs would be more resistant to anti-VEGF agents than predominately EC lesions. Previous studies have demonstrated that circulating bone marrow-derived cells (for this proposal these cells will be referred to as vascular precursor cells (VPrCs contribute to CNV lesions. These cells are proposed to migrate to the choroid and incorporate into CNV as endothelial cells (ECs) and various non-endothelial cells (vascular smooth muscle cells, fibroblasts, pericytes). This proposal will investigate the following hypothesis: A subset o nvAMD subjects display an increased prevalence of circulating VPrC cells that are predetermined to become non-EC cells, and this increase in non-EC cells is correlated to a need for frequent reinjections of intra-vitreal anti- VEGF agents in these subjects. To do this, th following aims will be undertaken. Aim 1: Demonstrate that purified CD34+ cells isolated from subjects with nvAMD display the characteristics of VPrC cells in culture. Aim 2: Demonstrate that the circulating VPrC populations, defined as purified CD34+ cells, can directly display markers of non-EC cells, and these cells are predetermined to become non-EC cells. Aim 3: Demonstrate that increased prevalence of VPrC-CD34+ cells with non-EC marker expression correlates with refractoriness to anti-VEGF therapy in nvAMD subjects. This research will dissect mechanisms of anti-VEGF refractoriness and begin to understand how patients could benefit from adjunct therapy for CNV with drugs that target non-EC cells in the lesion.